Fertilisation Flashcards
overview of fertilisation
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Describe the Acrosome
Developed from Golgi apparatus.
Bounded by membranes.
Contins various enzymes.
Membrane bound initially.
Changes occur to acrosomal membrane which allow contents to be released.
When/how does go through Sperm maturation
Sperm undergo maturation in the epididymis (under stimulation by androgens)
Further fluid absorption (100-fold).
Secretions include fructose, proteins and glycoproteins.
Results in transfer of proteins into and onto the sperm plasma membrane; increase in membrane fluidity.
What do sperm acquire during maturation?
They acquire ability to swim progressively
More rigid flagellum
cAMP content of tail increases
When are sperm able to fertilise an egg?
Once undergone capacitation.
Sperm are not able to fertilise an egg until they have undergone capacitation
WHat is capacitation induced by?
When does capacitation occur?
Induced by various components in female tract
This takes place predominantly within the female tract (proteolytic enzymes, cholesterol ‘sinks’ and higher ionic strength)
What are two main characteristics do fully capacitated sperm exhibit?
1) hyperactivated motility
2) changes in sperm plasma membrane properties that subsequently allow the acrosome reaction to happen. (i.e further membrane changes)
Why capaitation of sperm?
Why?
Enables sperm more responsive to signals from oocyte.
Sperm able to undergo the acrosome reaction which is essential for fertilisation
WHat follows on from capacitation?
WHat is capacitation of sperm?
Acrosome reaction follows on from capacitation.
What?
Stripping or modification of surface glycoproteins
Changes in surface charge; reduction in membrane stability through loss of cholesterol and formation of lipid rafts
How does capacitation occur?
How?
Mechanisms not fully understood; active research
Sperm cytoplasm becomes more alkaline - increase in bicarbonate conc.
Higher pH increases calcium permeability and hence intracellular calcium concentration. Subsequenct second messenger activation.
Results in increased adenylate cyclase activity and therefore cAMP production.
Activation of spermatozoal protein kinase A (PKA).
Downstream phosphorylation (including flagellum proteins); signalling pathways.
Hyperactivated sperm motility
Once sperm undergone compacitiation they become hyperactivated.
Involves high amplitude, asymmetrical beating of the flagellum
Helps the sperm meet its target
Aids sperm penetration of the zona pellucida
What are the difference movements of hyperactivted sperm?
The circling high-curvature (C) and helical (H) motility patterns correspond to the transitional trajectories and retain a sufficient symmetry to permit forward sperm movement. The star spin (S) type of motility, characterized by very low linearity, was generally recognized and appeared as an important predictor of success in in-vitro fertilization. Human sperm hyperactivation is not constant and, even over the course of 1 s, spermatozoa change their motility pattern (biphasic, B).
Helps sperm meet egg.
Finding the egg
In humans, over 99% spermatozoa do not even enter the cervix
Survival times: oocytes 6-24 hours after ovulation; sperm 24-48 hours in female tract
Sperm swim through uterus and into oviduct; cilia may help
Oocyte and cumulus cells release chemoattractants to aid sperm
Once fertilised Oocyte moves down the oviduct by muscular contractions and beating cilia
Acrosome reaction
WHen does it happen?
Described as the ‘terminal phase of the capacitation process’
Only occurs when sperm meets the egg (therefore doesnt happen to all sperm).
Occurs after sperm head binds to zona pellucida; zona proteins responsible for inducing reaction. Induced by proteins on zona pellucida.
Sperm acrosome swells; the acrosome membrane fuses with the sperm plasma membrane. Results in acrosomal contents being released.
Acrosomal vesicle undergoes exocytosis
Release of hyaluronidase and exposure of acrosin on sperm membrane.
Fertilisation- generalised acrosomal process
- Sperm makrd contact with egg
- Acrosome reacts with zona pellucica (releases contents).
- Acrosome reacts with perivitelline space
- Plasma membranes of sperm and egg fuse.
- Sperm nucleus enters eg
- Cortical granules fuse with egg plasma membrane, which renders the vitelline layer impenetrable to sperm. Sperm nucleus enters egg.
Zona pellucida - describe the 4 glycoproteins
The zona pellucida is composed primarily of four glycoproteins: ZP1, ZP2, ZP3 and ZP4
ZP1 is a structural protein; cross-links others
ZP2 contains sperm-binding domain necessary for oocyte-sperm recognition and penetration of the zona pellucida. Also responsible for the primary block to polyspermy
ZP3, complexed with ZP4, is involved in primary sperm-egg binding.
6 steps of Gamete binding
Hyaluronidase from acrosome digests cumulus cells and exposes acrosin, a protease on inner membrane of sperm
Primary binding: Sperm membrane binds zona pellucida via ZP3 (complexed to ZP4) and a species-specific complementary binding partner on the sperm, (species specific).
Secondary binding: Sperm inner acrosomal membrane binds to zona pellucida via ZP2 on zona and acrosin on sperm
Acrosin digests zona pellucida and oocyte plasma membrane
Adhesion of sperm equatorial region and oocyte membrane
Penetration of sperm head into oocyte
Oocyte activation and sperm/oocyte fusion.
Following sperm/oocyte fusion, PLCζ is released into the cytoplasm of the oocyte, where it facilitates the hydrolysis of membrane-bound PIP2to DAG and IP3, triggering Ca2+release from intracellular Ca2+stores, leading to Ca2+induced Ca2+release and oocyte activation.
Oocyte activation - calcium concentration.
Increase in cytosolic calcium concentration; calcium oscillations
Stimulates cortical granule release (includes Ovastacin - essential for block of polyspermy)
Protein kinase C (PKC) stimulates phosphorylation of other proteins essential for development of the conceptus
Block to polyspermy - how is polyspermy prevented?
Ca2+ stimulates fusion of zygotic cortical granules with oocyte plasma membrane and release of contents into perivitelline space
Enzymes including Ovastacin act on zona pellucida which hardens; inactivation of sperm receptors through ZP2 cleavage: Block to polyspermy
Completion of fertilisation: Calcium
Calcium pulses stimulate resumption of meiosis; second polar body is extruded and female pronucleus forms
Completion of fertilisation: Sperm
Sperm nucleus decondenses; protamines are replaced by histones; male pronucleus forms
Completion of fertilisation: Pronuclei
Pronuclei come together; DNA replication occurs; pronuclear membranes break down and replicated chromosomes align on mitotic spindle ready for first cleavage division
Syngamy
Combination of two genomes.
